This invention relates to a surface radiation detector, and specifically an instrument for detecting low energy radiation that may be contaminating a surface of an object.
This invention has particular application in radio-pharmaceutical laboratories of hospitals or other laboratories where carbon-14, tritium or other similar radioactive materials may be used. When such radioactive products are used, it is necessary to test for surface contamination from time to time to insure that the working area is free of radioactive components. One prior method for conducting a surface contamination test is to take a moistened towel or and wipe the surface. The towel is then placed in a scintillation counter where any radioactivity picked up is measured. Since all surfaces that are subject to contamination must be tested, this can be a time consuming process. A hand-held surface radioactive detector or counter would save a great deal of time in the examination of such surfaces.
Several radioactive detectors or counters may be used for this purpose, but many are very expensive. A gas proportional counter is inexpensive and may be used, provided the radioactive contamination can be brought into the enclosed gas chamber where it can be detected. Carbon-14 or tritium are both beta emitters, and the energy level of the beta particles is very low; for tritium, the maximum energy is in the order of up to 18 KEV, and for carbon-14, the maximum energy is 155 KEV. To measure low energy beta, an open window counter may be employed, such as the device shown and described in U.S. Pat. No. 3,603,831, issued Sep. 7, 1971. An essentially open window is necessary because any obstruction between the source and the interior of the counter will stop or absorb the beta particles.
A gas proportional counter requires a source of counter gas, often a mixture of argon and carbon dioxide or argon and methane, such as a commercially available mixture known as P10. Water vapor and oxygen must be excluded from the detection chamber. Counter gas, however, is relatively expensive, so it would be desirable to conserve the gas as much as possible, but the detector is basically open to gas flow through the screen.
Also to be taken into consideration is the characteristics of the radioactive emissions from tritium. The mean free path of beta emissions from tritium are short; the maximum path is about 5 mm, the mean is under 1 mm, which means that one-half the beta emissions are less than that. The advantage of an open screen gas proportional counter is that once a beta particle passes the screen, it will be counted. The problem is in getting the screen close enough to the beta particles that they can pass through the screen and be counted. If the screen is not close enough to the surface under test, then the beta particles simply will not pass through the screen.
Further, the area covered by the detecting instrument preferably should be at least 100.sup.2 cm; however, many surfaces on which radioactive contamination may be found are simply not flat, and unless the entire area of the screen is close enough to the surface to measure low energy radiation, then the desired area of inspection will not be achieved.